Turkey’s prime minister Ahmet Davutoğlu, who is due to visit London for talks with David Cameron.
Photograph: AP

More than two dozen prominent writers, including David Hare, Tom Stoppard, Hari Kunzru, William Boyd, Ali Smith, Sarah Waters and Monica Ali, have called on David Cameron to urge the Turkish government to halt its crackdown on freedom of speech.

The campaign by writers’ associations English PEN, Wales PEN Cymru and Scottish PEN comes amid growing persecution of journalists, writers and publishers in Turkey, with hundreds arrested, prosecuted or sacked over the past year.

In a letter to the prime minister, signed by 25 writers, the PEN branches warn that “over the past five months, intimidation, threats and even physical assaults against journalists, writers and publishers have become the norm”.

It asks Cameron to urge the Turkish prime minister, Ahmet Davutoğlu, who is visiting London next week, “to ensure that his government acts in accordance with Turkey’s obligations to respect the right to freedom of expression”.

Kunzru, one of the letter’s signatories, said the Turkish government had “revealed a deeply authoritarian streak”.

Among the incidents of intimidation highlighted by PEN are two attacks on the offices of the newspaper Hurriyet,including one supported by a serving Turkish MP; the raid and seizure of Koza Ipek Media, known for being critical of President Recep Tayyip Erdoğan; and the detention of three journalists working for Vice News. The letter warns that “independent journalism is now on its knees”.

The authors also raise concern for Can Dündar, editor-in-chief of Turkish daily Cumhuriyet, and Erdem Gül, the paper’s Ankara bureau chief, who were arrested on espionage and national security charges on 26 November.

The charges against the pair, who continue to be held in pre-trial detention, relate to an article and video published on the Cumhuriyetwebsite on 29 May 2015 about Turkey’s intelligence agency providing arms to Islamist rebel groups in Syria.

The authors write: “We are appalled that both journalists may face a maximum life sentence for the charge of divulging state secrets, a sentence of 20 years for espionage, and 10 years for membership of a terrorist organisation, for a story that the authorities appear to have accepted is true.”

Director of English PEN Jo Glanville said: “This is a crisis for democracy in Turkey that has repercussions not only for its citizens but for the rest of the world.

“Given the pressures that face the region, it’s more important than ever that Turkish journalists and writers are allowed to exercise their right to freedom of expression and inform public debate.”

The Republican People’s Party (CHP) said the government was using anti-terrorism laws to stifle press freedom, with 156 journalists arrested in the country last year.

Sezgin Tanrikulu, deputy chairman of the CHP, said on Monday that seven Turkish media companies came under investigation in 2015, 484 legal actions were launched against journalists, and 774 were fired for reasons related to their work.